MATHEMATICAL-MODEL OF TEMPERATURE-SENSITIVE PLASMID REPLICATION

The copy number of a series of plasmids constructed at Odense Universi ty is regulated by the lambda P-R/P-RM promoters and the temperature-s ensitive cI857 repressor. At low temperatures, these plasmids exhibit the low copy number of the parent plasmid R1 (5-6 per cell). At high t emperatures, the plasmids exhibit runaway replication, reaching copy n umbers of greater than 1000 per cell. A detailed mathematical model of the temperature-sensitive replication of these plasmids has been deve loped incorporating three features: replication of the parent plasmid, regulation of the lambda P-R/P-RM promoters by the cI repressor, and thermal denaturation of the c1857 repressor. Models of the first two o f these features have been described by others. We revised and extende d those models, described the thermal denaturation of the cI857 repres sor. and integrated these features to give a comprehensive model of te mperature-sensitive plasmid replication. Model predictions were compar ed to experimental measurements of both steady-state copy numbers as a function of temperature and the change in copy number following tempe rature shifts up and down. The model accurately describes the qualitat ive behavior of the system and gives reasonable quantitative results. This is particularly significant since all the parameter values used i n this model were determined independently: that is, there was no adju stment of parameter values to match our experimental data. The regulat ory system that gives rise to the temperature-sensitive replication of these plasmids is widely used in biotechnology applications, so the e lements of the model related to this regulation should be applicable t o a wide variety of systems. (C) 1994 Academic Press, Inc.